For the detection of gamma and X-ray radiation, in particular in CT, dual-energy CT, SPECT and PET systems, use is made inter alia of direct-converting detectors, based on semiconducting materials, such as CdTe, CdZnTe, CdZnTeSe, CdTeSe, CdMnTe, InP, TIBr2, HgI2. However, with these materials the effect of polarization arises, in particular with a high radiant flux density necessary for CT devices.
Polarization means the reduction in the detected count rate at high photon or radiant fluxes. This polarization is caused by the very low mobility of the charge carriers, especially the electron voids or holes, and by the concentration of intrinsic impurities in the semiconductor. The polarization thus arises due to the reduction in the electric field because of localized charges linked to impurities, the charges acting as capture and recombination centers for the charge carriers generated by the X-ray radiation. The result is a reduction in the service life and mobility of charge carriers, which in turn leads to a reduction in the detected count rate at the high radiant flux density.
The polarization of the semiconductor material changes during a measuring process. This change in the electric field in turn results in a change in the measured pulse heights and thus also affects the count rate of the semiconductor detector, also referred to as drift. Thus due to the polarization the maximum detectable radiant flux of a direct converter is limited. In particular in the case of a high radiant flux density necessary for CT devices the effect of the polarization is amplified. For this reason it has not until now been possible to convert high radiation densities, as used especially in computed tomography, directly into electrical pulses. The detector signal can no longer be directly linked to the attenuation of the object to be measured.
This problem has not as yet been completely solved. One possible solution is to largely forestall the polarization of the semiconductor material by irradiating the detector with additional X-ray radiation, this additional irradiation being performed immediately before a measuring process. However, this method is not suitable for patient operation, since the patient would be exposed to an additional dose. Due to the additional X-ray irradiation before the measuring process a pre-biased state of the detector is set, and the semiconductor material is thus deliberately polarized, so that both calibration measurements and actual measuring processes can be performed.
Another possible solution is to perform measuring processes with a constant current feed value of the detector. This means the quasi-Fermi levels can be kept constant. This is done for example by generating additional charge carriers in the semiconductor material before the incidence of the X-ray radiation to be detected. If the actual X-ray irradiation starts, the impurities are already populated with charge carriers, corresponding to the state of equilibrium under X-ray irradiation. The polarization of the semiconductor material is equalized. The electrical field thus stays constant during the measuring process and a unique link can be created between an attenuation by the examined object and the count rate of the detector.
In another possible solution the semiconductor material is irradiated with infrared radiation. This irradiation results in a conditioning of the detector which is similar to irradiation with X-ray radiation, the IR radiation being easy to manage and harmless for the patient. In the past, it has been known to irradiate the semiconductor material through the planar cathode. However, direct irradiation of the semiconductor material is difficult, since the direct radiation path onto the semiconductor material is restricted by the scattered radiation grid. For uniform irradiation there consequently remains only a narrow gap between the bottom of the scattered radiation grid and the top of the semiconductor. No solution to this problem is known to date.